Process for reducing edge roughness in patterned photoresist

ABSTRACT

A process for reducing roughness from a surface of a patterned photoresist. The process includes exposing a substrate having the patterned photoresist thereon to a vapor, wherein the vapor penetrates into and/or reacts with the surface of the photoresist. The substrate having the patterned photoresist thereon is then heated to a temperature and for a time sufficient to cause the surface of the photoresist to flow and/or react with the vapor wherein the surface roughness decreases. Optionally, the substrate is exposed to radiation during the process to increase the etch resistance of the photoresist and/or facilitate the reaction of the vapor with the surface of the photoresist.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a Divisional of Ser. No. 09/712,443 filed on Nov.14, 2000 now U.S. Pat. No. 6,582,891.

This application is a Continuation-in-part application of, and claimspriority from, U.S. patent application Ser. No. 09/452,878 entitled,“UV-Assisted Chemical Modification of Photoresist”, filed on Dec. 2,1999 now U.S. Pat. No. 6,503,693, hereby incorporated by reference inits entirety.

FIELD OF THE INVENTION

The present invention provides a process for reducing the roughness fromthe surfaces formed in photoresists after a relief image is patternedand developed in the photoresist. In particular, the process involvesexposing the patterned photoresist to a vapor to reduce roughness fromthe exposed surfaces of the photoresists.

BACKGROUND OF THE INVENTION

In the course of manufacturing integrated circuits, a semiconductorwafer is typically coated with a photoresist layer. Generally, thephotoresists used for this layer are organic photosensitive films usedfor the transfer of images and are typically exposed through a patternedphotomask to a source of activating radiation such as ultraviolet lightto form a latent image. The photomask has areas opaque and transparentto the activating radiation such that the photomask image is transferredto the photoresist layer. The latent image is then developed to yield apositive or negative relief image that is identical to or very similarto the photomask pattern. The photoresist layer may be positive ornegative acting depending on the photoresist chemistry and developerschosen. The wafer is then further processed until the desired integratedcircuit device is fabricated. The processing may be carried outaccording to a number of processing steps, including etching, doping andthe like. Each module of the process typically requires the use of atleast one patterned photoresist layer to transfer the desired propertiesand designs to the integrated circuit.

The relentless drive towards smaller sub-micron feature sizes in orderthat more and faster components may be included in a chip of a givensize has created numerous technical challenges for microlithographicprocessing. The technical challenges include developing new materialsand processes capable of accurately and reproducibly creating thedesired small lithographic structures. Utilizing smaller wavelengths ofradiation have made it possible to resolve smaller features. However, inorder to transfer those small features into the integrated circuit it isnecessary to make modifications to the photoresist chemistry.

Traditional I-line resists, based on novolac resins, have functionedwell for critical dimension feature sizes down to 0.35 um. However,novolac resins are not suitable for use at the smaller wavelengths sincethese materials strongly absorb radiation from the wavelengths usedbelow 365 nm. As such, in order to ensure the requisite opticaltransparency, new materials, such as poly(hydroxystyrene) andpolyacrylates, are generally used as resins for the shorter wavelengths.

DUV photoresists resists are optimized for exposure to radiation havinga wavelength at about 248 nm. DUV resists are generally based on achemical amplification mechanism because it offers high contrast, goodsensitivity and demonstrated ability to resolve patterns at 0.25 um andsmaller. DUV photoresist formulations typically include a resin and aphotoacid generator (PAG). Commonly used resins include modifiedpolyvinyl phenol polymers or polyvinylphenol/acrylate copolymers inwhich the phenol or carboxylate groups are partially “blocked” orprotected by moieties that can be chemically cleaved. As previouslymentioned, these systems are typically based on an acid catalyzedmechanism wherein a strong acid is generated upon exposure to aphotoacid generator compound present in the photoresist formulation toactivating energy. The acid catalytically and chemically cleaves theprotected group. As a result, a dissolution differential then existsbetween exposed (deprotected polymer) and unexposed (protected polymer)regions. It is this catalytic mechanism that is primarily responsiblefor the high sensitivity of these systems.

Photoresists optimized for exposure to radiation at 193 nm employ avariety of other protected polymers that necessarily contain few or noaromatic groups due to the strong absorption of 193 nm light to aromaticcompounds. For example, some of these photoresists are based on acrylatepolymers having various non-aromatic functional groups. Similar to DUVphotoresists, these resists also utilize an amplification mechanism. ThePAG chemistry of 193 nm resists is essentially the same as that of DUVresists.

At feature sizes below about 0.10 um it is anticipated that 157 nmwavelength light will be used for imaging. Photoimaging of these typesof resists will likely require a vacuum since oxygen is known tostrongly absorb at this wavelength. As such, a new generation ofchemically amplified photoresist polymers will have to be designed tomeet the challenging demands of lithography at this wavelength.

Inherent to all photoresists lithographically patterned is a phenomenonknown as edge roughness, nanoedge roughness, or line edge roughness.Edge roughness refers to the irregularities or coarseness present on thesurfaces of the photoresist after patterning and development. Thegreatest severity of edge roughness is typically observed on thesidewalls of the patterned photoresist and has a significant impact onthe critical dimension budget. Edge roughness becomes increasinglyimportant as one transitions to smaller feature sizes. For example, thecritical dimension error budget targeted by lithographers for 100 nmlines is ±7 nm. If sidewall roughness for a 100 nm line causesvariations of only 4 nm on either side of the targeted edge, then overhalf the critical dimension (CD) error budget has been used up. Edgeroughness thus consumes an increasingly larger portion of the CD budgetas the critical dimension of the feature shrinks. Accurate transfer ofthe roughness into the subsequently etched pattern will contribute tounacceptable variations in electrical performance.

There has been a great deal of effort directed towards understanding theroot causes of edge roughness. The cause of edge roughness can beattributed to numerous factors. For example, edge roughness can beattributed to inadequate mask quality, poor resist performance, pooraerial image contrast near the limits of optical resolution, the plasmaetch process and numerous other sources recognized by those skilled inthe art. One particular class of edge roughness is associated with topsurface imaging (TSI) processes based on selective silylation of theexposed resist film. Well-known problems with this approach include poorsilylation contrast of the resists and low glass transition temperature,and hence higher mobility of the silylated polymer. However, in mostcases other than TSI systems, edge roughness can be attributed tocharacteristics of the resist itself or to the lithographic processingof the image, i.e., post-exposure bake and developing conditions.

Another causal factor contributing to edge roughness is excessive aciddiffusion generated upon exposure of the photoacid generator to theactivating radiation at the edges of the latent image pattern, which maycause partial deprotection of the resist polymer beyond the edges. Thisis the main reason that so much effort has been directed at PAG designand understanding the kinetics of the deprotection reaction.Post-exposure bake conditions which drive diffusion of thephotogenerated acid, can therefore have a very significant effect onedge roughness.

Another cause of edge roughness is believed to be uneven dissolution ofthe resist during development. Developer process conditions have beenfound to have a large influence on edge roughness with different effectson isolated and dense features often resulting. Several researchers haveattributed this type of edge roughness to the resist polymer compositionand non-homogeneous regions formed during lithographic patterning anddevelopment. For example, researchers have determined that the formationof polymer clusters during exposure and post expose bake (PEB) leads toedge roughness due to the differing solubility of the clusters andnon-clustered polymers (T. Ha et al., Proc. SPIE, vol. 3999, 66 (2000)).Similarly, research has indicated that for some resists, phaseseparation of protected and de-protected polymers significantly enhancesedge roughness (Q. Lin et al., Proc. SPIE, vol. 3999, 23 (2000)).Research has also revealed a strong dependence of edge roughness onresist polymer molecular weight and polydispersity (T. Yoshimura et al.,Jpn. J. Appl. Phys., vol. 32, 6065 (1993)). More recently, researchershave discovered that not only these properties of the polymer, but alsomonomer ratio and the nature of the PAG influence edge roughness (S.Masuda et al., Proc. SPIE, vol. 3999, 25 (2000)).

Clearly, as noted, edge roughness can result from the complex interplayof a variety of factors. The demands on the resist formulator andlithographers to optimize several important properties simultaneously,such as resolution, sensitivity, environmental stability, etchresistance, and line edge roughness, inevitably result in compromisingsome properties at the expense of others. There clearly exists a greatneed for post lithography processes that could uniformly address all thecausal factors for edge roughness.

Annealing the patterned photoresist features has been evaluated as apost lithography method to reduce edge roughness. In negative toneresists, annealing has been found to be effective for reducing edgeroughness. However, it is reported that annealing deleteriously affectsthe feature and significantly changes the critical dimension size. (G.Reynolds, Ph.D. Dissertation, University of Wisconsin-Madison (1999)).Negative tone resists typically include a mechanism wherein thephotoresist crosslinks upon exposure to activating energy and becomesinsoluble in developer.

Annealing of chemically amplified, positive tone resists have also beenevaluated. However, positive tone photoresists resist are moresusceptible to flow deformation in an annealing process. Furthermore,thermal deprotection of these resists can occur at higher temperatures;causing release of the volatile blocking groups and concomitantshrinkage of the resist features.

Thus, there exists a need for a process for reducing the degree of edgeroughness in patterned resist features without significantly affectingthe critical dimensions of the feature. It is desirable that the processbe extendable to those photoresists used to pattern features less than250 nm, i.e., photoresists optimized and sensitive for exposure toactivating radiation at wavelengths of 248 nm, 193 nm, 157 nm and thelike. Moreover, it is desirable that the process be amenable to reducingedge roughness in positive tone photoresists as well as negative tonephotoresists. Importantly, the process should be economical, easilyimplemented and not dependent upon the source or causal factors of theedge roughness.

SUMMARY OF THE INVENTION

The present invention is generally directed to a process for reducingroughness from a surface of a patterned photoresist. The processincludes exposing a substrate having the patterned photoresist thereonto a vapor, wherein the vapor penetrates into and/or reacts with thesurface of the photoresist. The substrate having the patternedphotoresist thereon is then heated to a temperature and for a timesufficient to cause the surface of the photoresist to flow and/or reactwith the surface wherein the surface roughness decreases. Optionally,the substrate is exposed to radiation during the process to increase theetch resistance of the photoresist and/or facilitate the reaction of thevapor with the surface of the photoresist. The invention is especiallysuitable for use with those photoresists used for imaging feature sizesless than about 250 nm.

In one embodiment, the process includes exposing a substrate having apatterned photoresist thereon to a vapor, wherein the vapor penetratesinto the surface of the photoresist. The substrate is heated prior to,simultaneous with or after exposure of the vapor, to a temperature andfor a time sufficient to cause the surface of the photoresist to flowwherein the surface roughness decreases. The vapor lowers a glasstransition temperature at the surface of the photoresist relative to aglass transition temperature of a bulk of the photoresist that is freefrom exposure to the vapor. The vapor is selected from a material thatis miscible with or at least partially miscible with at least onecomponent in the photoresist. The temperature for heating the substrateis below a glass transition temperature for a bulk of the photoresistwherein the bulk of the photoresist is free from exposure to the vapor.

Optionally, the process further includes exposing the photoresist toactivating radiation for a time and energy sufficient to increase anetching resistance of the photoresist prior to, simultaneous with orsubsequent to exposing the photoresist to the vapor. The radiation thatis used to expose the photoresist has a wavelength in the ultravioletrange, x-ray range or includes electrons generated from an electronbeam, or the like.

The vapor can be selected from a material that is reactive ornonreactive during the process. Preferably, the vapor is generated froma material with a boiling point less than about 200° C. at standardatmospheric conditions. Examples of suitable non-reactive vapors includeketones and esters such as acetone, methyl ethyl ketone, butyl acetate,ethyl lactate and propylene glycol methyl ether acetate.

In another embodiment, the process for reducing edge roughness includesexposing a substrate having a patterned photoresist thereon to areactive vapor. The reactive vapor diffuses into the surface of thephotoresist and lowers the glass transition temperature at the surfaceexposed to the vapor. The substrate is heated to a temperature and for atime sufficient to cause the surface of the photoresist to flow whereinthe surface roughness decreases. During the process, the patternedphotoresist is exposed to an activating radiation prior to, simultaneouswith or subsequent to exposing the substrate to the vapor wherein theactivating energy reacts with the photoresist to generate a compound.The vapor reacts with the compound and adds mass to the photoresist. Thecompound is reactive with the vapor and is preferably, a free radical, aphotoacid generator, a photobase generator, or the like.

The reactive vapor can be generated from a wide range of materialsincluding but not limited, to vinyl ethers, epoxides, acrylonitriles,furans, coumarins, indenes, styrenes, acrylates, aryl halides,halosilanes, alkynes, alkenes, cyclic ethers and sulfur dioxide. Thereactive vapor reacts with the photoresist during and/or after thermalflow of the surface to increase a glass transition temperature for thephotoresist surface relative to a glass transition temperature of bulkphotoresist wherein the bulk photoresist is free from exposure to thevapor. The reactive vapor is preferably selected to add mass to thephotoresist. Preferably, the reactive vapor is generated from a liquidwith a boiling point less than about 200° C. at standard atmosphericconditions. The inventive process can be used in the manufacture ofintegrated circuits.

Other embodiments of the invention are contemplated to provideparticular features and structural variants of the basic elements. Thespecific embodiments referred to as well as possible variations and thevarious features and advantages of the invention will become betterunderstood when considered in connection with the accompanying drawingsand detailed description that follows.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a cross sectional view of a vapor treatment chambersuitable for use in the present invention; and

FIG. 2 shows pictorial views of scanning electron micrographs showing across sectional view of a 250 nm line processed in accordance oneembodiment of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention is generally directed to a process for reducingedge roughness from surfaces of patterned photoresists. Edge roughness,also referred to as line edge roughness or nanoroughness, is hereinafterdefined as irregularities or coarseness present on the surfaces of thephotoresist after patterning and development. The greatest severity ofedge roughness is typically observed on the sidewalls of the patternedphotoresist and has a significant impact on the critical dimensionbudget. According to standard practice, there is typically employed bythose in the art a critical dimension budget of +/−10 percent of thetargeted feature size. For example, if the critical dimension featuresize are 250 nm nested lines, the budget accorded to this criticaldimension is +/−25 nm for each individual line. The budget representsthe permissible amount of deviation from nominal that would still allowthe manufacturer to produce an integrated circuit with the desiredperformance specifications. Thus, in the above noted example, if theimage transferred from the photomask to the photoresist is greater than275 nm, the budget would be exceeded and targeted device performancewould not be met.

Depending on the lithographic process parameters employed, edgeroughness can be controlled to some extent in the patterned photoresist.However, it is expected that some degree of edge roughness will bepresent after developing the latent image formed in the photoresist.Current lithographic processing technology does not provide anyprocesses for rendering smooth the surfaces of the patternedphotoresist. The amount of edge roughness is especially problematic inmanufacturing integrated circuits as one transitions to smaller targetedfeature sizes for improved performance. As the critical dimensionsdecrease, the overall percentage of surface roughness for the criticaldimension increases since the degree of roughness present on thesurfaces of the developed photoresist is relatively constant. Thepresent invention provides a process that decreases the degree ofsurface roughness present in the patterned photoresist withoutdeleteriously affecting the critical dimension. Consequently, theinventive process advantageously permits a larger process window toexist.

The invention is especially suitable for use with those photoresistsused for defining features with a critical dimension less than 0.25 um.

The process generally comprises exposing the patterned photoresist to avapor and then heating the substrate having the photoresist thereon to atemperature and for a time sufficient to decrease edge roughness at thesurfaces. Optionally, the vapor and/or photoresist may be exposed duringthe process to activating radiation to facilitate the process fordecreasing roughness from the patterned photoresist surfaces.

The vapor may be reactive or nonreactive with the photoresist dependingon the choice of material selected to generate the vapor. In the case ofa non-reactive vapor, the vapor penetrates into the surface of thephotoresist and is believed to lower the glass transition temperature(T_(g)) at the surface of the photoresist such that the surface T_(g) islower than a glass transition temperature for a bulk of the photoresist.That is, the surface T_(g) is lower than the T_(g) for the photoresistthat was not exposed to any vapor. Subsequent heating of the photoresistat a temperature at or above the surface T_(g) causes the surface of thephotoresist to flow and smooth out the surface irregularities. The flowof the photoresist can be controlled on a microscopic scale based on theparameters and materials selected for the process. That is, the depth ofpenetration of the solvent in the surface of the photoresist can becontrolled and limited to the depth of the surface of irregularitiessuch that subsequent flow of the exposed areas renders the surfacesmooth. It is preferred that the temperatures selected for selectivelyflowing the photoresist surface are below the T_(g) of the bulkphotoresist or are for a time such that flow of the bulk photoresist isminimal and does not deleteriously affect the targeted criticaldimension of the photoresist feature. The temperature used for heatingthe photoresist surface may be constant or variable. Typically, thetemperatures used for flowing the photoresist surface are from about 20°C. to about 250° C. The total time and temperature used is dependent onthe photoresist thermal properties and the determination thereof is wellwithin the skill of those in the art to optimize.

The particular components of the non-reactive vapor are selected bytheir ability to form a vapor at vapor forming conditions and theirability to penetrate into the surface of the photoresist. Accordingly,the vapor can be a single material or a mixture generated from a solid,liquid or gas. Preferably, the vapor is a solvent or co-solvent for thepolymer used in the photoresist. However, in some cases, it is desirableand advantageous to employ a material with which at least one of thecomponents in the photoresist is only marginally soluble or miscible. Ithas been found that this will help control the extent of vaporpenetration and subsequent plasticization. The solid, liquid and/or gasmaterials are exposed to a temperature and/or pressure sufficient tomaintain the selected material in its vapor phase. The range of vaporpressures suitable for use in the present invention is in a range fromabout 1 atm to about 760 atm. In practice, preferred materials areliquids with a boiling point less than 200° C. at standard atmosphericpressure. Examples of suitable solvents for forming the vapor include,but are not limited to, ketones such as acetone, methyl ethyl ketone,and esters such as butyl acetate, ethyl lactate and propylene glycolmethyl ether acetate. Other materials suitable for use in the presentinvention will be apparent to one skilled in the art in view of thisdisclosure.

Exposure to the vapor may be limited by time and/or by concentration(vapor pressure). The effect of such permeation and penetration into thesurface of the photoresist will be to selectively lower the T_(g) at thesurface via a plasticization of the polymer chains. Solventplasticization is a well-known phenomena known to those skilled in theart to lower the T_(g) of polymers. This in turn will allow the polymerchains to flow in the vapor-permeated region at a lower temperature thanthat of the bulk resist.

The use of an optional hardbake or photostabilization step to increaseetch resistance can be included in the process before, during or afterexposure of the photoresist to the non-reactive vapor. The hardbake orphotostabilization process results in an advantageous increase in etchresistance believed to be caused by an increase in crosslink densitywithin the photoresist and/or a decrease in free volume withinphotoresist. One such photostabilization process for increasing etchresistance is disclosed in U.S. Pat. No. 4,548,688, entitled “Hardeningof Photoresist”, which is incorporated herein by reference in itsentirety. The photostabilization process generally includes exposing thephotoresist to UV radiation at an elevated temperature. The elevatedtemperature at any instant during exposure to the radiation is keptbelow the glass transition temperature for the resist. Careful controlof the amount of UV radiation is required during photostabilizationsince it is possible, depending upon the photoresist chemistry, to causethe patterned photoresist to shrink. The UV radiation or other form ofenergy may have a constant or variable intensity typically ranging fromabout 10 mW to about 1W. Since most photoresists used for patterningfeatures below 0.25 um utilize a chemical amplification mechanism basedupon photogeneration of a strong acid to cleave protective groups inorder to effect a solubility differential for subsequent development, itis possible that photostabilization will generate an amount of acidwithin the patterned photoresist that will cause the loss of functionalprotective groups present on the polymer used for the photoresist. Theprotective groups are typically volatile and do not remain in thephotoresist whereby the patterned photoresist shrinks as a result of aloss in molecular weight due to loss of the protective groups. Thus,careful control of the amount and wavelength of radiation is required tominimize shrinkage.

In another embodiment of the process, the patterned photoresist isexposed to a reactive vapor. As with the non-reactive vapors, thereactive vapor initially diffuses into and begins to plasticize asurface of the photoresist. Heating the photoresist to a temperature ator above the surface Tg and below the bulk photoresist Tg during theinitial diffusion of the reactive vapor will cause the photoresistsurface to selectively flow and decrease the roughness associated withthe surface. However, unlike the non-reactive vapors, the reactive vaporis selected from a material that will self react under certainconditions or react from contact with one or more components of thephotoresist.

Like the nonreactive vapor, the particular components of the reactivevapor are selected by their ability to form a vapor at vapor formingconditions and their ability to penetrate into the surface of thephotoresist. Accordingly, the reactive vapor can be a single material ora mixture generated from a solid, liquid and/or gas. Preferably, thereactive vapor is soluble to some extent with at least one component inthe photoresist. The solid, liquid and/or gas material is exposed to atemperature and/or pressure sufficient to maintain the material in itsvapor phase without undergoing any self initiated reaction. It is onlyafter the reactive vapor has diffused into the surfaces of thephotoresist that reaction will occur. The range of vapor pressuressuitable for use in the present invention is between from about 1 atm toabout 760 atm. In practice, preferred materials are liquids with aboiling point less than 200° C. at standard atmospheric pressure.

The reactive vapor can be one of a number of different reactive typematerials. For example, the vapor can be selected from materials thatare acid sensitive. That is, the reactive vapor upon contact with anacid will undergo self-polymerization or will react with one or more ofthe components found in the photoresist. As recognized in the art,chemically amplified photoresists are commonly used for defining featuresizes less than 0.25 um. Most chemically amplified photoresist systemsare based on an acid catalyzed mechanism in which a strong acid isgenerated from a photoacid generator (PAG) during exposure to activatingenergy. The acid generated catalytically cleaves protective group(s)from the base polymer in the photoresist to cause a differential indissolution behavior between exposed and unexposed regions duringsubsequent development. As such, the exposed areas of photoresist can bedifferentiated and selectively developed from the unexposed areas. Traceamounts of residual acid are present in the patterned photoresist afterdevelopment and can function as a catalyst for vapor polymerization suchthat a depth of the vapor diffusion into the photoresist surface islimited.

Optionally, the patterned chemically amplified photoresist may beblanket exposed to activating radiation to cause further generation ofacid within the patterned photoresist. The reactive vapor reacts andpolymerizes as it diffuses into the photoresist thereby limiting thedegree of penetration Accordingly, the acid generated within thephotoresist offers a self-limiting mechanism for causing slight flowinitially at the resist surface in order to cause a smoothing of theedge roughness and then halting the flow before gross deformation of theresist takes place.

While the applicability of the invention to chemically amplified systemsbased on acid generation has been discussed, the invention is notlimited to photoresists with acid amplification mechanisms. Somephotoresists include photobasic compounds that generate a basic compoundduring lithographic processing. In this case, the vapor is selected tobe reactive with the basic compound generated. Moreover, exposure ofradiation to the photoresist can cause the generation of free radicalswithin the photoresist itself that can then be used to react with thereactive vapor. Chain scission of the polymer used in the photoresistmay generate radicals, e.g., phenyl radicals. The radicals can thenreact with the vapor. Still yet, the initiation of polymerization ofmonomers can be caused by UV irradiation alone. In either case,absorption of the reactive vapor into the photoresist surface followedby UV irradiation will cause the formation of free radicals that caninitiate polymerization, thereby fixing the adsorbed vapor into theresist structure.

As noted, monomers are suitable for use as a reactive vapor in thepresent invention. Examples of monomers suitable for acid-catalyzedpolymerization include monomers having at least one vinyl group. Forexample, methyl vinyl ether, ethyl vinyl ether, acrylonitrile, methylmethacrylate, tert-butyl vinyl ether, 2,3-dihydrofuran, phenyl vinylether, styrene, indene, and benzofuran. Vinyl ether compounds areespecially suitable for polymerization initiated by a strong acidcatalyst. Other vinyl compounds suitable for use in the presentinvention will be well within the skill of those in the art in view ofthis disclosure. Prior, simultaneous or subsequent generation of strongacid from photoinduced decomposition of the pliotoacid generator in sometypes of chemically amplified photoresists initiates polymerizationand/or addition to the photoresist polymer. In the case of di- ormulti-functional monomers some crosslinking may also occur.

Another class of monomers capable of undergoing acid-catalyzedpolymerization and/or addition to the photoresist polymer consists ofepoxy-functional chemicals. Examples of epoxies suitable for use in thepresent invention include, but are not limited to,(2,3-epoxypropyl)benzene, ethylene oxide, and propylene oxide. Otherepoxies will be apparent to those skilled in the art in view of thisdisclosure.

Examples of such monomers that readily undergo free radicalpolymerization include various acrylates, styrenes, and acrylonitriles.Other materials suitable for free radical polymerization in accordancewith the present invention will be apparent to those skilled in the artin view of this disclosure.

Advantageously, it has been found that the glass transition temperatureof the surface can increase depending on the choice of reactive vapor.As the patterned photoresist is exposed to the reactive vapor, the vaporpenetrates and diffuses into the surface of the patterned photoresist.Initially, the diffusion of vapor into the photoresist surfacesdecreases the Tg at the surface where exposure occurred. However, as thevapor reacts with and polymerizes upon contact with the residual acidand/or free radical, an increase in Tg can be made to occur. As such,the depth of reactive vapor penetration into the surface can becontrolled since polymerization may prevent further penetration anddiffusion of reactive vapor. Advantageously, by careful selection of thereactive vapor, the reaction with the photoresist can increase the etchresistance of the photoresist by increasing the amount of carbon tohydrogen present in the photoresist. Those skilled in the art willrecognize that an indication of etch resistance can be determined by theratio.

The process of using a reactive vapor may further include a hardbake orphotostabilization process to increase the etch resistance, i.e.,increasing crosslinking density of the photoresist features prior tovapor treatment and/or minimizing the free volume in the photoresist. Aspreviously noted, one of the problems with photostabilization ofchemically amplified resists systems is the propensity for shrinkage.Use of a reactive vapor can counteract shrinkage since the process canbe optimized to control the amount of vapor diffused into thephotoresist. If shrinkage is severe the amount of reactive vapordiffusing into the resist can be increased to cause the resist to swell.For example, the vapor pressure can be increased. As such, crosslinkingand reactive swelling may be made to take place simultaneously in amanner where the actual feature dimensions never undergo appreciablechange in spite of the chemical changes taking place, or they may takeplace sequentially, e.g., crosslinking (and shrinking) first, thenswelling back to the original size with the reactive solvent. In anotherprocess meeting the requirements of this invention, any of a variety ofvinyl type monomers which are solvents for the photoresist phase(s) andwhich are capable of polymerization and/or addition to reactive groupson the photoresist polymer, are diffused into the photoresist.

A distinct advantage to the use of monomers that are capable ofself-polymerizing is that there are no stoichiometric limitations.Complete reaction can take place regardless of the quantity of materialdiffused into the resist, thus providing greater control over criticaldimensions. In contrast, reactive vapors that react stoichiometricallywith functional groups on the photoresist polymer may not be sufficientto counteract shrinkage since the numbers of functional groups presenton the photoresist polymer are limited.

The process is applicable to decreasing surface roughness fromphotoresist features wherein the surface roughness is caused by pooraerial image quality, optical effects or the like, or wherein thesurface roughness is caused by the formation of non-homogeneous polymerdomains near the surface of the photoresist. The main components ofphotoresists are polymers or copolymers that have different protectivemoieties designed to deprotect under certain conditions. For example,DUV photoresists are comprised of polyvinyl phenols wherein the phenolis blocked with an acid sensitive protective group or is apolyvinylphenol acrylate copolymer with acid sensitive protectivegroups. Removing the protective groups during lithographic processinggenerates the phenol, or carboxylic acid or the like, which rendersthese types of polymers soluble in aqueous basic developer. Developersused in the development of the latent image are typically chosen basedon the bulk dissolution properties for the exposed and unexposedphotoresist. Near the interface between the exposed and unexposedregions there is often a boundary layer consisting of protected polymerchains and (partially) deprotected polymer chains which were not removedby the developer. These immiscible polymer domains form clusters thatresult in surface roughness. The photoresist pattern is treated with areactive vapor capable of reacting with the de-protected groups on thepolymer and in the process re-protecting them. In this manner all of thepolymer chains within the resist features may now become miscible andupon appropriate application of heat will become homogenized. As aresult, heating at or above the Tg for the resist surface and below theTg for the bulk photoresist will cause concomitant flow at the surfaceto decrease roughness.

In many cases, even if a relatively smooth sidewall profile is obtainedafter exposure and development, the non-homogeneous quality of theresist results in local variations in resist erosion rates during thesubsequent plasma etch steps. This results in etch induced surfaceroughness. Depending on the choice of reactive vapor and processingconditions, the vapor can be chosen to crosslink within the photoresistto produce a more uniform etch resistance thereby reducing line edgeroughness induced by the plasma etch process. Also, the reactive vapormay be chosen so as to re-protect the functional groups in the polymerchains rendering all polymer chains homogenous and miscible.

In another embodiment all of the photoresist polymer may first bede-protected (via UV/thermal treatment) in order to confercompatibility, then treated with a reactive chemical. The (partially)deprotected polymer chains near the exposed photoresist surfaces willpossess acid groups in the salted form. The counter-ion, e.g.,tetramethylammonium cation, derives from the developer solution. It isdesirable to rid the polymer chains of the cations in order to renderthe polymer chains more compatible with the deprotected bulkphotoresist. In these cases, first treating the photoresist featureswith an excess of a mild acid, such as acetic acid, will render thepolymer neutral and compatible with the bulk polymer polymer. Ifdesirable, the salt by-product, e.g., tetramethylammonium acetate, canbe rinsed away with water. This treatment may or may not be followed upwith reactive vapor treatment.

A complementary method involves de-protection of all of the photoresistpolymer (readily accomplished via UV/thermal treatment) and thentreatment with a base such as ammonia. This will render the entirepolymer in the ionomeric form and at the same time swell the polymer tocounteract the shrinkage that occurs upon deprotection. Appropriatethermal treatment of the now homogeneous polymer will effect the surfacesmoothing. Properties of the resulting “salted” polymer can be tailoredby judicious choice of base and treatment conditions. In general,ionomers usually have a higher Tg and modulus than the correspondingneutral polymers, and so the resulting photoresist features may haveenhanced resistance to subsequent processes such as plasma etching orion implant.

Referring now to FIG. 1, there is shown a vapor treatment chambersuitable for use in the present invention that is generally designatedby reference numeral 10. The invention is not limited to any particularchamber construction in this or in the following embodiments. Thechamber 10, as shown, includes a receptacle 20 that contains a wafersupport 22, a vapor inlet 24, a vacuum port 26, a gas purge port 28 andan optional window 30. A radiation source 32 is proximate to the window30. The radiation source preferably emits light in the ultravioletregion. However, it is possible that other wavelength regions can beused, e.g., x-rays. The wavelength of the radiation source is dependenton the desired photoreactions. The window is made from a material thatis transparent to the emitted radiation. For example, a quartz window ispreferably used for emission of light with a wavelength selected fromthe ultraviolet region. The temperature of a wafer 34 placed on thewafer support may be thermally controlled by conventional means using anexternal controller. A vacuum pump 40 is connected to the vacuum port bymeans of a conduit 42. A gas source 44 is connected to the purge gasport 28 via conduit 46. The vacuum pump advantageously removes thestatic atmosphere, e.g., oxygen and/or air, from the receptacle 20 inthe event that the atmosphere in the receptacle causes deleteriouseffects on the desired reaction. Moreover, it has been found thatevacuating the chamber to a certain pressure level will facilitate vaporformation for chemicals that are liquids at ambient temperatures andpressures. In addition, for safety reasons, it is desirable to purge thereceptacle with a gas to remove any toxic or obnoxious gases and/orvapors present before it is vented to the atmosphere. As such, exposureto a user of potentially harmful and hazardous materials is prevented.Optionally a manometer (not shown) may be attached to the receptacle fordetecting the pressure within the chamber. In this way, through controlof the temperature and pressure of the entire system, a desired vaporpressure of a chemical treatment may be maintained in the chamber andaround the wafer. A tank 50 is connected by a heated conduit 52 to thevapor inlet 24. The tank 50 preferably includes conventional means forheating and pressurizing the tank for generating the vapor.

In operation, a semiconductor wafer 34 with patterned photoresistthereon is placed into the chamber 10 on the wafer support 22. Thepressure within the reaction chamber is reduced or increased to anamount sufficient to maintain the vapor in vapor form. The vapor is fedinto the chamber via the vapor inlet 24. Preferably, the vapor is heatedexternally in a heated conduit 52 connected to the chamber. It ispreferred that the vapor penetrate the surface of the photoresist to adepth sufficient to remove roughness present on the surface. Of course,depending on the degree of edge roughness the amount of vaporpenetration can be easily optimized by adjusting the pressure within thechamber, by changing the chemical constituency of the vapor and/or byvarying the duration of the exposure to the vapor. It has been foundthat limiting penetration to the depth of the irregularities will causethe irregularities to flow upon exposure to an elevated temperaturewhereby the surface roughness is decreased. The wafer is then heated ator above the surface Tg and below the Tg of the bulk photoresist to atemperature and time sufficient to decrease the degree of surfaceroughness initially present. The vacuum is then released and theprocessed wafers are removed from the chamber.

The following example is a detailed description of the methods ofpreparation and uses of the composition of the present invention. Thedetailed description falls within the scope of, and serves to exemplify,the more generally described methods set forth above. The example ispresented for illustrative purposes only, and is not intended to limitthe scope of the invention.

EXAMPLE 1

In the following example, a DUV photoresist was coated over a bottomanti-reflective coating on multiple 8″ silicon wafers. The bottomanti-reflective coating used in this example is known by the trade nameAR3 and is commercially available from the Shipley Company. Thephotoresist is available under the trade name M20G and is commerciallyavailable from the JSR Microelectronics, Inc. Each wafer was patternedwith a test reticle having various features using conventionallithography and developers. One wafer was cross sectioned andmicrographs of the cross section were obtained using scanning electronmicroscopy. FIG. 2A is a pictorial view of the micrographs showing across sectional view of 250 nm lines after lithography. The profilesclearly show a significant roughness on the sidewalls of the patternedlines.

Two patterned wafers were then exposed to approximately 275 mW/cm² froman H-Mod bulb for 30 seconds. During the UV exposure the temperature ofthe wafers was increased from about 80° C. to about 120° C. After UVexposure the wafers were heated to about 140° C. for 60 seconds. Onewafer was cross-sectioned and micrographs were obtained using scanningelectron microscopy. FIG. 2B is a pictorial view of a micrographic crosssectional view of the 250 nm lines after exposure to UV radiation andheat. A small reduction in roughness is observed.

The single wafer that was exposed to UV radiation and heat was thenexposed to a vapor generated from 2,3 dihydrofuran at approximately 250torr for about 90 seconds at 90° C. After exposure to the vapor, thewafer was then heated to 140° C. for 60 seconds. The wafer wascross-sectioned and micrographs of the cross section were obtained usingscanning electron microscopy. FIG. 2C is a pictorial view of themicrograph showing a cross sectional view of 250 nm lines afterlithography. The profiles clearly show a significant reduction inroughness on the sidewalls of the patterned lines. The reduction inroughness is greater than the roughness observed after UV radiation andthermal processing.

Many modifications and variations of the invention will be apparent tothose skilled in the art in light of the foregoing disclosure.Therefore, it is to be understood that, within the scope of the appendedclaims, the invention can be practiced otherwise than has beenspecifically shown and described.

We claim:
 1. A process for reducing roughness from a surface of apatterned chemically amplified photoresist, the process comprising: a)exposing a substrate having patterned chemically amplified photoresistresulting from imaging and developing processing thereon to a vapor,wherein the vapor diffuses into the surface of the patternedphotoresist; b) heating the substrate to a temperature and for a timesufficient to cause the surface of the patterned photoresist to flowwherein the surface roughness decreases; c) exposing the patternedphotoresist to an activating radiation prior to, simultaneous with orsubsequent to exposing the substrate to the vapor to form a compound,wherein the compound is reactive with the vapor; and d) reacting thevapor with the compound to form a polymer.
 2. The process according toclaim 1 wherein the compound is selected from the group consisting of afree radical, an acid, and a base.
 3. The process according to claim 1wherein the vapor is generated from a material selected from the groupconsisting of vinyl ethers, epoxides, acrylonitriles, furans, coumarins,indenes, styrenes, acrylate, aryl halides, halosilanes, alkynes,alkenes, cyclic ethers and sulfur dioxide.
 4. The process according toclaim 1 wherein the vapor reacts with the photoresist to increase aglass transition temperature for the photoresist surface relative to aglass transition temperature of bulk photoresist wherein the bulkphotoresist is free from exposure to the vapor.
 5. The process accordingto claim 1 wherein the vapor is 2,3-dihydrofuran.
 6. The processaccording to claim 1 wherein the radiation that is used to expose thephotoresist has a wavelength in the ultraviolet range.
 7. The processaccording to claim 1 wherein the radiation that is used to expose thephotoresist has a wavelength in the x-ray range.
 8. The processaccording to claim 1 wherein the radiation consists of electronsgenerated from an electron beam.
 9. The process according to claim 1wherein the vapor is generated from a liquid with a boiling point lessthan about 200° C. at standard atmospheric conditions.
 10. The processaccording to claim 1 wherein the vapor is a monomer selected from thegroup consisting of indenes, furans, vinyl ethers, epoxides, styrenes,acrylate, alkenes and alkynes.
 11. A process for reducing roughness froma surface of a patterned photoresist, the process comprising: a)exposing a substrate having patterned photoresist resulting from imagingand developing processing thereon to a reactive vapor, wherein the vaporpenetrates into the surface of the patterned photoresist; b) heating thesubstrate to a temperature and for a time sufficient to cause thesurface of the patterned photoresist to flow wherein the surfaceroughness decreases; and c) polymerizing the reactive vapor within thephotoresist by exposing the patterned photoresist to an activatingenergy whereby the vapor reacts in the patterned photoresist to form acompound.
 12. The process according to claim 11 wherein the reactivevapor is selected from a material that reacts with a functional group ona polymer in the patterned photoresist.
 13. The process according toclaim 11 wherein the reactive vapor is selected from a material thatreacts with a free radical in the patterned photoresist to form anonvolatile compound.
 14. The process according to claim 11 wherein thestep of polymerizing the vapor comprises contacting the reactive vaporwith an acid generated by the activating energy.
 15. The processaccording to claim 11 wherein the step of polymerizing the vaporcontacting the reactive vapor with a base generated by the activatingenergy.
 16. The process according to claim 11 wherein the reactive vaporis a vinyl ether.